One proposed fabrication technique for coating rigid magnetic disk memories is to sputter a ferro-magnetic material from a target onto a substrate to a controlled thickness and uniformity. This sputter coating is accomplished in an evacuated chamber having a controlled concentration of ionizable gas. Electrons moving in a confined region due to a magnetic field in the chamber ionize the gas in regions above the target and an electric field accelerates the charged ions toward the target with sufficient momentum to cause target material to be ejected when the ions collide with the target.
Other than ferro-magnetic materials can be used for sputter coating. Integrated circuits can, for example, be fabricated by sputtering a conductive layer or layers from a target onto a non-conductive substrate and then selectively etching the one or more conductive layers.
U.S. Pat. No. 4,629,548 to Helmer entitled "PLANAR PENNING MAGNETRON SPUTTERING DEVICE" discloses a Penning cathode for sputtering a material onto a substrate positioned relative to the cathode. Unlike a more conventional magnetron cathode, a Penning type cathode uses a cathode/anode structure and magnet combination that results in the magnetic field lines intersecting the cathode sputtering surface at generally right angles. If an ionizable gas in sufficient quantity is present in the vicinity of the cathode this causes ionizing electrons to spiral in the magnetic field and ionize the gas atoms. The charged gas ions are attracted to the cathode and impact the cathode to sputter off cathode material that then coats the substrate.
In the Helmer patent, two sputtering cathode targets and a single anode are spaced symmetrically about a center axis passing through an inner one of the cathode targets. As disclosed in FIG. 4 of the Helmer patent, the sputter surfaces of the two cathode targets are, at least initially, approximately co-planar. A thin cylindrical anode separates the two targets and extends beyond the co-planar surface of the targets to intersect the "lie of sight" co-planer surface shared by the two cathode targets.
FIG. 3 of the Helmer patent is a graph of current versus voltage for different anode configurations and as indicated in this graph, the voltage applied to the target material becomes essentially independent of the current at high voltages. FIG. 5 suggests that this independence between current and voltage is also experienced at different gas pressures. Since the measured current on the target is an indication of target erosion rate due to ion bombardment of the target, the Helmer construction results in a system wherein target erosion and therefore coating rate cannot be controlled (at least at high currents) by modifying the target potential.